Abstract
During the casting of ferromanganese alloys from electric arc furnaces into sand beds at temperatures of up to 1800°C a considerable amount of very brown fumes are generated when the alloy fume is oxidized in the atmosphere. The fume is difficult to capture because of the large flux of gas that is generated. Possible reasons for this flux include the high evaporation rate of Mn at elevated temperatures, the large surface area of the casting beds and the large thermal plumes over the furnace tapholes and casting beds. It has been found that the use of fine water sprays along the edge of the roof that covers the casting bed resulted in a significant reduction in visible emissions. This paper describes research into the kinetics of the fume to improve the design of the capture hoods, as well as the mechanism of suppression by the water sprays by using CFD analysis. It is shown that the oxidation reaction produces less than 20% of the energy content of the plume over the arc furnace taphole, and also that radiation heat transfer may play an important role in increasing the energy content of the taphole plume. The capture of fume particles by fine spray droplets is shown to have limited efficiency, while the heat sink that is caused by evaporation does not materially contribute to the circulation of fume through the spray. It is postulated that the increased moisture content of the air over the casting beds may be instrumental in reducing the oxygen partial pressure or in the formation of an oxide layer, both of which would reduce metal evaporation and, therefore fume formation. The exact mechanism requires further investigation
Highlights
Eramet Sauda, in Norway, operates two ferromanganese furnaces producing high carbon ferromanganese (HCFeMn) as well as a Manganese Oxygen Refining (MOR) unit to produce medium and low-carbon ferromanganese (LCFeMn)
A study was conducted to investigate whether a fine water spray can suppress fume emissions from casting bed operations and, mitigate Mn3O4 pollution without enclosing the sand beds or significantly increasing the fume capture capacity
To fully understand the fine water spray system’s role in suppressing secondary emission fumes, it is necessary to understand the mechanism involved in the formation of these fumes, as well as possible energy contributors to the fumes’ energy content
Summary
Eramet Sauda, in Norway, operates two ferromanganese furnaces producing high carbon ferromanganese (HCFeMn) as well as a Manganese Oxygen Refining (MOR) unit to produce medium and low-carbon ferromanganese (LCFeMn). Lee et al (2005) considered various reaction-limiting factors for the fume formation in the casting of the high carbon FeMn produced in the arc furnace They found that the Mn in the melt is essentially lost through evaporation and oxidation to form MnO mist, which is further oxidised to Mn3O4 particulate. The change in circulatory pattern and/or the increased humidity of the air is believed to be involved in the enhanced rate of formation of an oxide layer on the molten metal, thereby reducing the rate by which fumes form This will be discussed further under Heat Transfer Effects on this page. Assuming spherical droplets and uniform distribution of the fume particulate in the air, the efficiency of particle capture, as illustrated graphically in Figure 4 (Kopita 1955). The effectiveness of the spray system can be found from:
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